Page 429 - Marine Structural Design
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Chapter 22 Material Selections and Damage Tolerance Criteria 405
expert judgement should be used to quantify the fatigue strength improvement due to the
modification of the weld profile.
22.3.4 Modification of Residual Stress Distribution
By using the following methods, the undesirable tensile residual stresses found at the weld can
be modified to obtain desirable compressive stresses at the weld toe:
Stress Relief
Various fatigue tests on simple small plate specimens indicate that improved fatigue strength can
be obtained by stress relief due to post-weld heat treatment (PWHT). However, plate and
stiffening elements of continuous systems rarely require stress relief. It is also doubtful that a
complex structural detail with built in constraints can be effectively stress relieved.
Compressive Overstressing
Compressive overstressing is a technique in which compressive residual stresses are introduced
at the weld toe. Experimental results and analytical work demonstrate the effectiveness of pre-
overstressing, but the procedure to be implemented does not appear to be practical for most
marine structures.
Peening
Peening is a cold working process intended to produce surface deformations with the purpose of
developing residual compressive stresses. When impact loads on the material surface would
cause the surface layer to expand laterally, the layer underneath prevents such surface layer
expansion, creating the compressive residual stresses at the surface. Typical peening methods
are hammer peening, shot peening, and needle peening.
22.3.5 Discussions
Fatigue strength improvement techniques are time consuming and costly and they should be
applied selectively. Comparison of different techniques allows assessment of their effectiveness
and cost. The recommended improvement strategies depend on the characteristics of the (global
and local) structure and the preference for one technique over others is based on effectiveness,
cost and fabrication yard characteristics.
Some of the comparisons of various approaches available, which improve fatigue strength of
welded details, are the following:
Full profile burr grinding is preferable to toe burr grinding or disc grinding only, because it
results in higher fatigue strength even at a substantial cost penalty.
Disc grinding requires the least time and cost. However, it produces score marks
perpendicular to the principal stress direction, making this technique less effective than
others.
Using a high-pressure abrasive water jet (AWJ) process for controlled erosion of the weld
toe area can be as effective as grinding. Its simplicity, speed and non-utilization of heat
make controlled erosion very promising.
A wider weld pool makes plasma dressing less sensitive to the position of the electrode
relative to the weld toe, compared with TIG dressing. Therefore, the fatigue strength
improvement obtained from plasma dressing is better than that obtained from TIG dressing.